Field of the Invention
The present invention relates to a pulse wave velocity measurement method and system as well as an imaging device, and more particularly to a technique for measuring a pulse wave velocity of a human body based on a time difference between pulse waves in different parts of a human body.
Description of the Related Art
A pulse wave velocity (PWV) has conventionally been used as one of the evaluation indexes in the circulatory system such as arteriosclerosis. Examples of pulse wave velocity measurement methods in practical use include a method of calculating a pulse wave velocity based on a time difference between pulse waves (pulse wave propagation time) measured in different parts of a human body and a distance between the parts measured at two points. In addition, there has been disclosed a technique for calculating blood pressure based on a time difference between pulse waves (pulse wave propagation time) measured at two points: wrist and finger, using a correlation between pulse wave velocity and blood pressure (see Japanese Patent Application Laid-Open No. 2011-104208; PTL 1).
These pulse wave velocity measuring devices are, however, not suitable for everyday use because these devices require a measurement sensor to be attached to two parts such as a pair of a neck region and a thigh region and a pair of an upper arm and an ankle joint, each pair being far away from each other. These devices may be available for everyday use by general users as long as the pulse wave sensors are attached to relatively easily attachable parts such as wrist and finger, but involve problems of requiring high time resolution, increasing cost, increasing measurement error depending on the condition of the joints, and the like.
In addition, there has been disclosed a method of measuring based on a time difference between an incident wave component and a reflected wave component contained in a pulse wave measured at one point of parts of a living body (see Japanese Patent Application Laid-Open No. 2003-10139; PTL 2). According to this, the pulse wave sensors can be relatively easily attached and may be used for everyday use by general users. However, the reflected wave component contained in the pulse wave is greatly affected by a change in blood vessel condition due to a change in posture or the like, thus causing a problem of being difficult to accurately measure.
Meanwhile, Japanese Patent Application Laid-Open No. 2007-319246 (PTL 3) discloses a technique in which a camera mobile phone is used to image a hand finger in a state of being in contact with an opening of the camera and to detect a temporal variation in the finger image, thereby to measure the pulse rate.
In addition, Japanese Patent Application Laid-Open No. 2010-264095 (PTL 4) discloses a technique in which an infrared camera is used to detect a temperature of a surface (skin) of a living body and to extract frequency data corresponding to a frequency component of a frequency band corresponding to a heart rate of a living body from a temporal variation in temperature information about the living body, and then based on the frequency data, the heart rate of the living body is measured.